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[FD] IBM WebSphere deserialization of untrusted data

Security Advisory           @ Mediaservice.net Srl
(#02, 07/10/2016)           Data Security Division

         Title:	IBM WebSphere deserialization of untrusted data
   Application:	IBM WebSphere 7,8,8.5,9
   Description:	The application server deserializes unstrusted data
                when the WASPostParam cookie is present. This can lead
                to a DoS via resource exhaustion and potentially remote
                code execution.
       Authors: Federico Dotta <federico.dotta AT mediaservice.net>
                Maurizio Agazzini <inode AT mediaservice.net>
 Vendor Status: Fixed (PI62375)
 CVE Candidate: The Common Vulnerabilities and Exposures project has
                assigned the name CVE-2016-5983 to this
    References: http://lab.mediaservice.net/advisory/2016-02-websphere.txt

1. Abstract.

When using custom form authentication, IBM WebSphere automatically sets
the cookies WASPostParam and WASReqURL when the LtpaToken2 session
cookie is invalid. The data contained in these cookies is used when the
user logs in again to retrieve the last visited page and related parameters.

The WASPostParam is a serialized Java object that is deserialized
without checking the object type. This behavior can be exploited to
cause a denial of service and potentially execute arbitrary code.

The objects that can cause the DoS are based on known disclosed payloads
taken from:

- https://gist.github.com/coekie/a27cc406fc9f3dc7a70d

Currently there is no known chain that allows code execution on IBM
WebSphere, however new chains are discovered every day.

2. Example Attack Session.

The attack can be reproduced as follows:

- create an application with custom form authentication
- after user login, the LtpaToken2 is set by the application server
- make a HTTP GET request that contains the WASPostParam cookie with one
of these contents:

    * 01_BigString_limited_base64.txt: it's a string object; the server
      will reply in a normal way (object size similar to the next one).
    * 02_SerialDOS_limited_base64.txt: the application server will
      require about 2 minutes to execute the request with 100% CPU
    * 03_BigString_base64.txt it's a string object; the server will
      reply in a normal way (object size similar to the next one).
    * 04_SerialDOS_base64.txt: the application server will require an
      unknown amount of time to execute the request with 100% CPU usage.

3. Affected Platforms.

This vulnerability affects the following versions and releases of
IBM WebSphere Application Server and priors versions:

 - Liberty

4. Fix.

Apply Interim Fix PI62375

5. Proof Of Concept.

See websphere_payload.zip and Example Attack Session above.

6. Timeline

20/08/2016 - First communication sent to IBM PSIRT (psirt at us.ibm.com)
22/08/2016 - IBM Response, PSIRT Advisory 6345 assigned to the bug
05/10/2016 - Communication from IBM with fix information (PI62375)
07/10/2016 - Security Advisory released

Copyright (c) 2016 @ Mediaservice.net Srl. All rights reserved.

Maurizio Agazzini                     CISSP, CSSLP, OPST
Senior Security Advisor
@ Mediaservice.net Srl                Tel: +39-011-32.72.100
Via Santorelli, 15                    Fax: +39-011-32.46.497
10095 Grugliasco (TO) ITALY           http://mediaservice.net/disclaimer

"C programmers never die. They are just cast into void"

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